Steam generators such as the steam generators, of pressurized-water-cooled nuclear reactors, comprise an outer jacket of cylindrical overall shape arranged vertically in the nuclear reactor building, i.e., with the axis of the outer jacket vertical.
The steam generators of pressurized-water nuclear reactors allow the feedwater to be heated and turned into steam by exchanging heat with the pressurized water that cools the nuclear reactor and which flows through the tubes of a heat-exchange bundle. The tube bundle is arranged inside a bundle wrapper of cylindrical overall shape which is arranged coaxially inside the outer jacket.
The tubes of the bundle are fixed at their ends into a tube plate so that they open, at a first end, into a first part of a water box of the steam generator, and at a second end, into a second part of the water box of the steam generator. The water box of the steam generator allows the pressurized water from the nuclear reactor vessel, in which the core which consists of fuel assemblies is arranged, to be distributed, and allows the pressurized water which has flowed through the heat-exchange tubes to be recovered, so that the recovered pressurized water can be sent back to the nuclear reactor vessel.
Steam generator feedwater is introduced into the outer jacket and is routed as far as an inlet part of the heat-exchange bundle, towards the bottom of the bundle and bundle wrapper. The feedwater then flows from the bottom upwards inside the bundle wrapper in contact with the external surface of the tubes, so that it heats up and vaporizes and ends up in the form of steam towards the top of the outer jacket of the steam generator. The steam recovered in the top of the steam generator is sent to the reactor turbine.
The feedwater is generally introduced into the top of an annular space formed between the tube bundle wrapper and the outer jacket or between the bundle wrapper and a skirt delimiting a flow space communicating with an end part of the bundle that consists of the ends of the cold legs of the tubes, i.e., the legs of the tubes via which the reactor cooling water exits once it has been used to heat the feedwater and turn it into steam.
By using a feed space delimited by a skirt, the feedwater can be preheated by flowing in contact with the bundle wrapper and in contact with the cold legs of the tubes, which legs are separated from the hot legs by a vertical dividing wall directed across a diameter over part of the height of the bundle.
In any event, the feedwater introduced into the steam generator jacket flows from top to bottom in an annular space directed vertically and axially as far as near the bottom of the bundle wrapper.
To achieve good steam generator efficiency and satisfactory operating conditions, it is necessary for the flow of feedwater to be distributed in the circumferential direction of the annular feed space of the steam generator.
To that end, it has been proposed that use be made of a manifold in the overall shape of a torus which is arranged inside the outer jacket of the steam generator vertically in line with the upper part of the annular feedwater supply space. The manifold is connected to a feedwater feed pipe passing through the outer jacket of the steam generator, and comprises means of distributing and guiding the feedwater, these means being spread out in the circumferential direction of the annular feedwater supply space. The means of distributing and guiding the feedwater may consist of tubes in the shape of an inverted J, the vertical straight leg of which is fixed to the manifold, and the loop of which points downwards, towards the upper part of the annular feed space. The distribution in the circumferential direction and the flow rate passing through each of the J-shaped tubes makes it possible to obtain a satisfactory distribution of the feedwater in the circumferential direction of the annular space. However, this device has the drawback that the jets emerge from the J-shaped tubes at high speed, which disturbs the flow of water making its way towards the annular feed space.
It has also been proposed that this feedwater supply device be adapted to steam generators with preheating that comprise an economizer consisting of a feedwater guide skirt communicating with the end of the cold legs of the tubes of the bundle. In this case, the guide skirt, which is in the shape of a cylindrical sector, delimits with the bundle wrapper an annular space that extends over a circular arc of less than 180.degree. around the steam generator bundle wrapper. The manifold then consists of a portion of a torus extending over less than 180.degree. around the bundle wrapper, vertically in line with the upper part of the annular feed space. This device has the same drawbacks as the device in which the manifold consists of a jacket in the shape of a complete torus completely surrounding the bundle wrapper, because the feedwater leaving the manifold is also distributed using J-shaped tubes. It has been proposed that the J-shaped tubes be extended in the vertical direction, inside the annular feedwater supply space. This arrangement does not, however, fully solve the problems of turbulence and makes the mechanical design of the steam generator more complicated on account of the length of the outlet legs of the J-shaped tubes.
U.S. Pat. No. 5,396,948 proposes that the feedwater supply manifold be a spillway consisting of a channel section in the overall shape of a torus which is open at its top, or in the overall shape of a portion of an open torus, combined with feedwater guide walls directed downwards in the upper part of the annular space. The feedwater is brought into the manifold, by a feed pipe passing through the outer jacket of the steam generator and opening into the manifold. The feedwater fills the channel-section manifold up to the level of a spillway edge over which the feedwater flows into a flow space delimited by the guide walls. The spillway to a certain extent distributes the flow of feedwater around the circumference of the annular space, but it is very difficult to determine precisely what this distribution will be as a function of the spillway feed conditions.
Another drawback of the device comprising a spillway is that it is not easy to ensure that the spillway will be durable enough when subjected to high mechanical stresses, of the water-hammer type, or in an accident situation, for example if a feedwater pipe were to burst. It is therefore necessary to design components with which to construct the distribution device which are extremely strong and are made from very thick sheeting.